The experiment was performed in the Entomology Laboratory at China-Pakistan Joint Research Centre, University of Sargodha, Pakistan.
Aphid species
The parthenium weed, Parthenium hysterophorus L. (family: Asteraceae) and wheat plant, Triticum aestivum, (Family: Poaceae) were grown in plastic containers (10 cm diameter) at 25 ± 2 °C, 65 ± 5% RH and a photoperiod of 14:10 (L:D). Plants were irrigated with tap water, and pot weights were recorded twice each day to maintain the soil water contents which were kept the same for both plant species. The nymphs and adults of L. erysimi from wheat plants and A. gossypii from parthenium weed were collected directly from the field, released on the respective host plants and the cultures were maintained up to the 3rd generation.
Culture of H. convergens
The initial culture of H. convergens was initiated by collecting adults from the field and maintained in plastic jars (20 cm length and 15 cm diameter) with an abundant supply of aphid species. The culture was maintained separately on each aphid species. The rearing jars were provided by a crumpled paper to support as an oviposition site. The eggs laid were collected daily, transferred to clean Petri dishes and allowed to hatch. The culture was maintained at the laboratory conditions of 25 ± 2 oC and 65 ± 5% RH. Newly emerged larvae of H. convergens from the stock were the ones used in the experiments.
Life table parameters
To study the pre-imaginal development and survival, groups of about 40 eggs of H. convergens were obtained from the adults reared on the 2 different aphid species that placed separately in clean Petri plates. The egg incubation period was recorded at 12 h intervals. On hatching, larvae were fed on A. gossypii and L. erysimi separately. The experimental unit consisted of falcon centrifuge polypropylene tube (12 cm in length and 3 cm in diameter) containing 1 predator larva and aphid diet that was provided daily. First and second larval instars of H. convergens were provided by 10 1st and 2nd nymphal instars of each aphid species and later larval instars were provided by 20 3rd and 4th instars of each host aphid. Development of larval and pupal durations was recorded at 12 h intervals. The adults of H. convergens were selected from the corresponding experiment with the immature stages. Additional culture was also maintained to harvest a sufficient number of adult females when required. The adult pairs of H. convergens were isolated, and kept in transparent plastic jars containing 1–2 branches of each host plant; dipped in plastic vials containing water. Similarly, the adult pairs were fed daily on L. erysimi and A. gossypii. The laid eggs were separated daily for each couple. Data were recorded daily to determine the fecundity rate and adult longevity. The fecundity rate, developmental duration, adult pre-ovipositional period (APOP), and total APOP based on an age-stage two-sex life table (Chi 1988) was determined by using the computer program TWOSEX-MSChart (Chi 2016). Age-specific survival rates and life expectancy were calculated according to Chi and Liu (1985) and Chi and Su (2006), respectively.
Data analysis
The developmental duration and population parameters were calculated using TWOSEX-MSChart program. The bootstrapping technique with 100,000 replications was used to minimize the variation in the results for calculating the mean and standard error of the population (Efron and Tibshirani 1993). Using raw data, the stage mean, age-stage-specific survival rate (Sxj), age-stage reproductive value (Vxj), age-stage-specific fecundity (fxj), age-stage life expectancy (Exj), age-specific survival rate (lx), age-specific fecundity (mx), age-specific net maternity (lxmx), and life table parameters (Ro, net reproductive rate; r, intrinsic rate of increase; λ, finite rate of increase; and T, mean generation time) were calculated. The significant difference between means was estimated using quick paired bootstrapping (paired 1 by 1) function in TWOSEX-MSChart program (Chi 2018).
The age-specific survival rate (lx, mx, and Ro) was calculated as
$$ {\mathrm{l}}_x={\sum}_{j=1}^k{S}_{xj} $$
$$ {m}_x=\frac{\sum_{j=1}^k{S}_{xj}{f}_{xj}}{\sum_{j=1}^k{S}_{xj}} $$
$$ {R}_o=\sum \limits_{x=0}^{\infty }{l}_x{m}_x $$
Where k denotes the number of stages, x = age in days, j = stage, Ro (net reproductive rate) is the average number of offspring per female during its whole life cycle. It was calculated by the following equation:
The intrinsic rate of increase (r) (Goodman 1982), finite rate of increase (λ), and mean generation time (T) was calculated as
$$ {\displaystyle \begin{array}{c}\sum \limits_{x=0}^{\infty }{e}^{-r\left(x+1\right)}{l}_x{m}_x=1\\ {}\lambda ={e}^r\\ {}T=\ln\;Ro/r\end{array}} $$
The life expectancy (Exj) was referred as the expected life of an individual of age x and stage j was calculated by the equation suggested by Chi and Su (2006):
$$ {\mathrm{E}}_{xj}=\sum \limits_{i=x}^{\infty }.\sum \limits_{y=j}^{\beta }{\acute{\mathrm{s}}}_{iy} $$
Where śiy was the probability that individuals of age x and stage j will survive to age i and stage y, and was calculated by assuming ś = 1.
The reproductive value (Vxj) was calculated by the equation suggested by Tuan et al. (2014):
$$ {V}_{xj}=\frac{e^{r\left(x+1\right)}}{S_{xj}}\sum \limits_{i=x}^{\infty }{e}^{-r\left(i+1\right)}\sum \limits_{y=j}^{\beta }{\acute{\mathrm{s}}}_{iy}{f}_{iy} $$